Power amplifiers with complementary pair driver stages

ABSTRACT

A TRANSISTOR AMPLIFIER OF WHICH THE OUTPUT STAGE HAS TWO TRANSISTORS DRIVEN BY DARLINGTON COUPLED DRIVER TRANSISTORS WHICH RECEIVE THEIR INPUTS FROM A CURRENT LIMITING STAGE AND INCLUDING A SECOND STAGE SUPPLYING THE INPUT OF THE   FIRST STAGE AND HAVING AN OUTPUT IMPEDANCE WHICH IS LESS THAN THE FIRST STAGE INPUT IMPEDANCE.

Feb. 9, 1971 CQLLINSQN 3,562,658

7 POWER AMPLIFIERS WITH COMPLEMENTARY PAIR DRIVER STAGES Filed April 1, 1969 5 Sheets-Sheet 1 I Vac FIG. 1. Tr3

PRIOR ART 77*2 5 INPT f FIG. 2.

PRIOR ART Feb. 9,-1971 J. D. COLLINSON 3,562,658

POWER AMPLIFIERS WITH COMPLEMENTARY PAIR DRIVER STAGES Filed April 1, 1969 3 Sheets-Sheet 2 Feb. 9, 971 J. D. COLLINSON 3,562,658

POWER AMPLIFIERS WITH COMPLEMENTARY PAIR ISRIVER STAGES Filed April 1, 1969 3 Sheets-Sheet s United States Patent 3,562,658 POWER AMPLIFIERS WITH COMPLEMENTARY PAIR DRIVER STAGES John Dulm Colliuson, Otley, Yorkshire, England, assignor to The Rank Organisation Limited, London, England, a British company Filed Apr. 1, 1969, Ser. No. 811,919 Claims priority, application Great Britain, Mar. 29, 1968, 15,112/ 68 Int. Cl. H03f 3/18 US. Cl. 330-17 9 Claims ABSTRACT OF THE DISCLOSURE A transistor amplifier of which the output stage has two transistors driven by Darlington coupled driver transistors which receive their inputs from a current limiting stage and including a second stage supplying the input of the first stage and having an output impedance which is less than the first stage input impedance.

This invention relates to transistor power amplifier circuits and particularly to output-transformerless audio power amplifier circuits including a pair of output transistors each with a driver stage.

In a conventional output-transformerless power amplifier two output transistors of like polarity are connected in series with the DC. supply. In order to simplify the DC. connection of the driver stages and to obtain phase reversal complementary driver transistors are used, also connected in series with the DC. supply. The bases of the driver transistors can have the same (or nearly the same) DC. potential of Vcc/2 and can, therefore, be driven from a single transistor. With the simplest arrangement the whole of the A.C. output appears in series with the driver input, the driver/output stage has tfull negative feed-back, and the overall gain approaches unity. It is more effective 'to spread the negative feedback over earlier stages as well so that it is usual to bootstrap the output to the driver input and, therefore, remove all negative feedback from the driver/output stages.

The object of the invention is to produce economically an improved amplifier of the above type with a measure of self protection against external faults, with low distor'tion, and with high efficiency.

According to the invention there is provided a transistor power amplifier circuit comprising an output stage having two power output transistors of the same or opposite polarity tor connection in series with a DC. supply, and connected directly together, the junction of the output transistors being for connection to an output terminal, a complementary transistor pair driver stage, each transistor of said driver stage pair being effectively connected in a Darlington circuit with a different one of said output transistors, a single transistor first stage preceding said driver stage of which the output of the first stage transistor is connected to the inputs of said driver stage, which is a current gain stage, and which is designed to limit the current drawn by said first stage to such a value that the maximum current drawn by the ensuing stages is insuificient to damage the transistors of said stages, and a second stage having a transistor therein and preceding said first stage, the mutual conductance of the transistor of said second stage being determined by a resistance in its emitter circuit, said resistance not being by-passed by a capacitor, the input impedance of said first stage being lower than the output impedance of the second stage.

Thus, an amplifier according to the invention is protected against external faults by limiting the output current by means of the current-limiting circuit means, but

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the amplifier does not suffer from the disadvantages of variation in the mutual conductance of the first stage because the latter is arranged as a current amplifier.

The invention also permits the elimination of emitter resistors in the output stage thus affording a reduction in power loss and a simple temperature compensation, by making the current gain largely independent of collector current, whereby the output stage quiescent current may be small and not critical.

The invention will now be described in more detail with reference to the accompanying drawings in which:

FIG. 1 shows the basic circuit of a known output transformerless power amplifier circuit;

FIG. 2 shows the basic circuit of FIG. 1, rearranged with a push-pull pre-driver stage in a known manner;

FIG. 3 shows an amplifier circuit in accordance with one embodiment of the invention; and

FIG. 4 shows an amplifier circuit in accordance with another embodiment of the invention.

It is convenient, first to consider the known circuit as shown in FIG. 1.

CALCULATION OF VOLTAGE GAIN In the arrangement of FIG. 1 each driver transistor Tr3/Tr4 is effectively connected in a Darlington circuit with it output transistor Tr5/Tr6. In the absence of Tr5 emitter resistor R and assuming Tr3 and Tr4 base source resistance is high there is no output stage current feedback. The pairs Tr3/5 and Tr4/ 6 act as current amplifiers into the load on alternate half-cycles. If a peak output current of I is required the peak base current of Tr3 will be Given a peak output current 4 a., (minimum) Me; of 50 and hfe of 20, the driver peak base current will be 4 ma.

If this is the maximum current required, Tr 10 can be made 4 ma. and the output stage will be protected against excessive current.

For a collector current of 4 ma., the gm of Tr2 will be about ma./v. or re =8i2. The overall gain from Tr2 base into the load will be with R =8Q and the typical values given above the voltage gain=1000 or 60 db.

DISTORTION For 50 watts into 89, the output voltage V0=20 v. If the sensitivity required is say 0.5 v. the voltage gain required will be 40 or 32 db, leaving 28 db for negative feedback. In an amplifier of this type, i.e. basically current driven, the distortion will arise from variations of hfe. The output transistor current gain falls oiT with increasing Ia, and (if the output transistors are similar) this will produce odd order harmonics. If the current gain of Tr3/5 does not equal Tr4/6, amplification of alternate half cycles will not be equal and even order harmonics will be produced.

As Tr2 is working nearly to current clipping (necessary to limit output current) its gm will vary considerably and it will be a source of even order harmonic distortion.

If the distortions should total say 10% then 28 db of feedback will only reduce this to 0.4%. This is too near a target of say 0.5%. A method of improving this could be to closely match the current gains in each half, and possibly to add some form of linearizing to the Tr2 stage.

3 DISTINCTION BETWEEN VOLTAGE AND CURRENT AMPLIFICATION A simple common emitter transistor amplifier has a high output resistance. This may be reduced by using a low collector load or by the use of negative feedback derived from the voltage output.

The input resistance is low as current and voltage is required to produce an output. This may be increased by deriving feedback from the current through the load (normally by an emitter resistor). This effectively raises the input voltage required at the same current, therefore the input resistance is increased.

The input resistance of a voltage amplifier stage is high compared with its source resistance. Conversely the input resistance of a current amplifier stage is low compared With its source resistance. The change between voltage and current operation can be said to occur at the point where input resistance equals source resistance.

IMPORTANCE OF SOURCE RESISTANCE IN EMITTER FEEDBACK .STAGE It should be noted that the amount of negative feedback over a stage with an unbypassed emitter resistor depends on the source resistance fromwhich it is fed. If the source resistance (Rs) is high compared with the input resistance (Rin) the gain will be the same as if the source impedance is low. However, if the source is high the input signal is attenuated by the ratio:

Rt'n Rs-l-Rin If Rs is low the signal is little attenuated, but the overall gain remains the same because the feedback controls it. As the attenuation decreases the internal gain increases and, therefore, there is more feedback.

If the feedback is required for some definite function, e.g. to reduce distortion or improve thermal stability, it is necessary to check that Rs is low compared With Rirz otherwise the feedback may be ineffective.

EFFECT OF OUTPUT STAGE EMITTER RESISTOR The frequency cut-off of a transistor output stage can be extended by a factor of three or four by operating it as a voltage amplifier as the low source resistance reduces the effect of the high effective input capacitance. With a low driver source and an output stage emitter resistor, local feedback is applied over the output stage as the output stage gain is now controlled by R. However, at low levels R no longer swamps re and cross-over distortion appears unless the quiescent current is very carefully controlled. If the quiescent current is such that re 25x10- I is less than R, and the source impedance is less than hfe (R-l-re), the emitter resistor helps to stabilize the quiescent current.

In the circuit of FIG. 1 the emitter resistor R is effective only in T15 emitter (assuming Tr3 source is low) as the source resistance of Tr6 is re (very high) of TM. If the DC. voltage at the junction of Tr5 emitter and Tr6 collector is stabilized by overall D.C. feedback to Vac/2, then Tr5 emitter resistor Will stabilize Tr6 also. If Tr6 Iq drifts the overall D.C. feedback will return it to the Iq of TrS in order to keep the mid-point to Vcc/ 2.

As the output current flows through Tr5 emitter resistor is current driven, i.e. high source resistance no emitter resistor, the quiescent current can be very small (sulficient to ensure there is some hfe at the crossover). If the quiescent current is very small there is no thermal runaway problem. Distortion at high currents will increase, but this is offset by the increased gain which can be used as feedback overall, and the reduced crossover distortion.

In the amplifier circuit shown in FIG. 2 all the transistors have to withstand the full output swing (i.e. Vce=Vcc). It should be more economical to concentrate the gain in the low level stages especially as a single transistor is adequate in place of a pair.

As an alternative to closely matching the current gains in each half of the output stages of the known circuit shown in FIG. 1 and the possible addition of some form of linearizing to the Tr2 stage, as already stated under Distortion, the circuit of each embodiment in accordance with the invention, introduces within the main feedback loop and ahead of Tr2, an additional stage Trl.

EFFECT OF ADDITIONAL STAGE The additional stage Tr1 will produce an amplifier of greatly improved characteristics as not only will the basic gain be substantially increased but, if Tr2 is operated as a current amplifier (depending on hfe instead of gm), the initial distortion will be lower. The Tr1 stage gm distortion can be low as the standing current can be large compared With the peak current demand, Tr2 still being the current limiting stage.

The embodiment of the invention shown in FIG. 3 will now be described.

A +75 v. supply is used with a 12 v. supply in order to simplify the bias network and also to allow a bigger negative driver swing.

Assuming the amplifier is aimed at giving a total distortion of less than 0.1% say 0.07%, and the initial distortion is 10%, 43 db of negative feedback will be required. A voltage gain of 32 db is required with feedback so that the basic voltage gain without feedback must be 75 db (or 5,500). Given that Tr2 lzfe=l00, Tr3 hfe=50, Tr5 hfe=20, R =8 and the hfe -hfe -hfe -R basic ga1n- +7161 Re +re will be 1459.

If TrlIc is 500 a. its re is about 50, therefore, R6 should be 100. As the gain with feedback is 40, R5 should be 3.9K. (This will give a quiescent current of 10 ma. in T15, this may be balanced by connecting a further 3.9K resistor R2 between +75 v. and Tr5 emitter).

For TrlIc of 500 a. and TrlVc of 6 v. R4 will be 12K. As Trle is effectively at earth potential the voltage across The emitter current of 500 ,aa. flows through this so R3=75 K. For convenience 68K is used.

The peak output current of 4 a. is

hfe -hfe at Tr3 base. Tr2 standing current should be 4 ma. and as Ic2R7 should be about 5.5 v. to agree with Tr1 collector, R7=1.5K, R8+R9=37.5/4 ma.=9.4K so that R8+R9= 4.7K could be suitable.

These component values are given by Way of example and one component will have to be variable to set the DC operating point. MRI+R10 provide small forward bias for the small Tr5/ 6 Iq required.

Certain additional components will be needed to ensure stability with various loads, e.g. a CR network across the output to limit high frequency gain with no load.

The embodiment of FIG. 4 is identical with the embodiment described in relation to FIG. 3 except that T15 and Tr6 are of opposite polarity with the driver transistors T13 and T14 appropriately connected as shown.

The invention is not limited to transistors of the type shown, and transistors of opposite conductivity type may be used, with suitable alteration of the polarity of the biassing courses.

I claim:

1. A transistor power amplifier circuit comprising:

(a) an output stage having two power output transistors connected directly together in series and for connection to a DC. supply;

(b) a complementary transistor pair driver stage of which each driver transistor is effectively connected in a Darlington circuit with a different one of Output transistors;

(c) a single transistor, current limited, current-gain stage having its output connected to the inputs of said driver stage and which limits the maximum current drawn by said driver and output stages; and

(d) a further stage having its output connected to the input of said current-gain stage and including a transistor having connected in its emitter circuit a resistor that is not by-passed by a capacitor and which determines the mutual conductance of said transistor, the output impedance of said further stage being higher than the input impedance of said current-gain stage.

2. A circuit according to claim 1, wherein the two power transistors of the output stage are similar and of the same polarity.

3. A circuit according to claim 1, wherein the two power output transistors are of opposite polarity.

4. A circuit according to claim 2, including a feedback loop from the output stage, said further stage being included in said feedback loop.

5. A circuit according to claim 3, including a feedback loop from the output stage, said further stage being included in said feedback.

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6. A circuit according to claim 2, wherein the collector of the transistor of the current-gain stage is connected to the inputs of the driver stage, and the emitter circuit includes a parallel RC circuit.

7. A circuit according to claim 3, wherein the collector of the transistor of the current-gain stage is connected to the inputs of the driver stage, and the emitter circuit includes a parallel RC circuit.

8. A circuit according to claim 2, including a feedback loop from the output stage, said further stage being included in said feedback loop and wherein the collector of the transistor of the first stage, is connected to the inputs of the driver stage, and the emitter circuit includes a parallel RC circuit.

9. A circuit according to claim 3, including a feedback loop from the output stage, said further stage being included in said feedback loop and wherein the collector of the transistor of the current-gain stage is connected to the inputs of the driver stage, and the emitter circuit includes a parallel RC circuit.

References Cited UNITED STATES PATENTS 7/1959 Lin 330--13 10/1966 Reifl'ln 33017X OTHER REFERENCES ROY LAKE, Primary Examiner L. J. DAHL, Assistant Examiner U.S. Cl. X.R. 330l3 

